Continuous noninvasive monitoring of carbon dioxide (CO2) levels in infants, particularly in a Neonatal Intensive Care Unit (NICU), is considered very important mainly in order to protect subjects such as infants from the complications of hypocarbia (less than the normal level of carbon dioxide in the blood) and hypercarbia (more than the normal level of carbon dioxide in the blood) and to avoid extra blood sampling which may cause anemia, discomfort, and pain. Noninvasive monitoring of carbon dioxide (CO2) levels typically refers to exhaled breath analysis also referred to as capnography.
Capnography is a common method of monitoring and optionally displaying the CO2 level(s), CO2 waveform(s) and/or other CO2 related parameters, such as End Tidal CO2 (EtCO2), in exhaled breath. Capnography also provides information relating to cell metabolism, blood perfusion, alveolar ventilation and other body functions or conditions, and may enable real-time diagnosis of patho-physiological abnormalities as well as technical problems related to ventilation. In intubated subjects (such as patients) capnography is performed by sampling exhaled breath around the exit of the endotrachial tube (ETT) at a sampling region between the proximal end of the ETT (close to the subject's mouth) and the ventilator circuit. In intubated small children and infants, however, capnography is not commonly used since it does not consistently provide satisfactory results. One of the reasons for the lack of satisfactory results in small children, infants and/or neonates especially neonates, relates to the use of uncuffed endotrachial tubes (ETTs) during their ventilation. A cuff is an inflatable balloon on the outer surface of the ETT used to hold the ETT in place and to close off the ventilated air exiting the ETT at its distal end (towards the subject's lung) from escaping around the tube outwards. In case of small children and/or infants, and especially neonates, cuffed ETTs are generally not used due to the risk of perforating or otherwise harming the gentle membrane of their trachea. The use of uncuffed ETTs results in ventilated air escaping around the tube during inhalation. Although this can be compensated for by changing the ventilator parameters, a greater problem arises during breath sampling (both in mainstream and sidestream capnography) which depends on the exhaled breath returning back through the ETT towards the sampling region. The uncuffed ETT allows the exhaled breath to return between the ETT and trachea, and out through the mouth without returning back through the ETT towards the sampling region. This is further exaggerated, since with neonates the ETT internal diameter is very small, typically between 2.5 to 4 mm, which imposes a large restriction, enhancing further the possibility of exhaled breath escaping round the tube. The small volume of exhaled breath (which is only a fraction of the low tidal volumes of infants and neonates) that does manage to return back through the ETT is also diluted with clean ventilated air (often present in a “dead space” of the ETT) which leads to difficult breath sampling and erroneous CO2 readings.
This problem is further enhanced in mainstream capnography, in which the required airway section is connected inline between the proximal end of the ETT (close to the subject's mouth) and the ventilator circuit. Thus, it adds more dead space, competes for tidal volume, and may also cause a kink in the ETT, especially in small premature infants. When a flow sensor is connected to the ETT, the use of mainstream capnography is even more cumbersome. There is thus a need in the art for methods, systems and apparatuses that would allow accurate CO2 monitoring, particularly in small children, infants or neonates.
High Frequency Ventilation (HFV):
In addition to the problems discussed hereinabove, which relates to CO2 monitoring, the ventilation itself, particularly in neonates, but also with children and adults, still suffers from significant difficulties. Some neonates cannot be adequately ventilated even with sophisticated conventional ventilation. Therefore respiratory insufficiency remains one of the major causes of neonatal mortality. Intensification of conventional ventilation with higher rates and airway pressures leads to an increased incidence of barotrauma. Especially, the high shearing forces resulting from large pressure amplitudes damage the lung tissue. High Frequency Ventilation (HFV) has been shown to resolve or at least ameliorate this issue in many cases.
High Frequency Ventilation (HFV) is a technique of ventilation that uses respiratory rates that greatly exceed the rate of normal breathing. There are three main types of HFV:                1) High frequency positive pressure ventilation (HPPV, rate 60-150 breaths/minute);        2) High frequency jet ventilation (HFJV, rate 100-600 breaths/minute); and        3) High frequency oscillatory ventilation (HFOV, rate 300-3000 breaths/minute).        
During conventional ventilation direct alveolar ventilation accomplishes pulmonary gas exchange. According to the classic concept of pulmonary ventilation an amount of gas reaching the alveoli equals the applied tidal volume minus the dead space volume. At tidal volumes below the size of the anatomical dead space this model fails to explain gas exchange. Instead, considerable mixing of fresh and exhaled gas in the airways and lungs is believed to be the key to the success of HFV in ventilating the lung at such very low tidal volumes. Among the advantages of high frequency oscillatory ventilation as compared to either conventional positive pressure or jet ventilation is its ability to promote gas exchange while using tidal volumes that are less than dead space. The ability of HFV to maintain oxygenation and ventilation while using minimal tidal volumes allows minimization of barotrauma and thus reduces the morbidity associated with ventilation.
Currently, two of the most important values that determine the respiratory therapy, such as HFV, is the Blood Gas CO2 (PaCO2) and the SpO2 (the amount of oxygen being carried by the red blood cell in the blood). In order to monitor the subject's gas concentration in the blood, however, a blood sample must be taken. Blood sampling involves pain, discomfort and risk of infection. Especially with neonates, since the volume of blood is very small, each blood test takes a measurable percentage of the neonate's blood. This dictates periodic blood transfusions, where each blood transfusion promotes a further danger to the neonate or other subject.
There is thus a need in the art for methods, systems and apparatuses that would permit and facilitate accurate measurement(s) of medical parameter(s) for the evaluation and control of HFV therapy in subjects, particularly, but not only, in small children, infants or neonates.